This invention relates to a process for removal of hexafluoropropylene dimers and other oligomeric contaminants from fluids.
Fluorinated fluids derived from hexafluoropropylene have many industrial uses, such as coolants for electronic devices (e.g., supercomputers), inert solvents and fluids, fire-extinguishing agents and as heat transfer agents. However, in the processes used in their preparation, or upon transient heating, many of these fluorinated fluids form oligomeric byproducts such as dimers. These dimers may be hazardous to persons handling the liquid or operating equipment containing the contaminated liquid, or may be reactive, and therefore undesirable, under conditions of use.
U.S. Pat. No. 3,696,156 describes a method of removing perfluoroolefin and perfluorochloroolefin impurities from saturated fluoroperhalocarbon compounds having two to six carbon atoms, by contacting the impure fluoroperhalocarbon in the vapor phase at about 180 to 250xc2x0 C. with alumina containing a basic alkali metal or alkaline earth metal hydroxide or oxide.
U.S. Pat. No. 5,233,107 describes a process for removing olefinic impurities from hydrogen-containing chlorofluorocarbons in the gas phase at 200 to 400xc2x0 C. over a zeolite. The contaminated higher boiling chlorofluorocarbons are preheated to convert the liquid to the gas phase in advance. The addition of 0.5 to 10% air or oxygen by volume to the process stream is recommended to keep coking at a very low level. One of the disadvantages of processes utilizing elevated temperatures is that they require handling hot gases contaminated with hazardous compounds. In addition, certain fluorocarbons are unstable and generate a variety of olefinic and aliphatic impurities at elevated temperatures especially in the presence of catalytic surfaces.
A system and method for purifying saturated fluoroperhalocarbon liquids by removing olefinic impurities therefrom has been disclosed in U.S. Pat. Nos. 5,300,714 and 5,507,941. Inorganic oxide, hydroxide, carbonate, or phosphate particles are used in the method.
England et al., J Fluorine Chem. 1981, 17, 265-288, describe reactions of amines with a dimer of hexafluoropropene and a perfluorovinyl sulfide prepared from hexafluoropropene. Anhydrous ammonia was added to a solution of hexafluoropropene dimer to form (1-amino-2,2,3,3,3-pentafluoropropylidene)propanedinitrile.
An organic amine-impregnated activated carbon composition, which preferably has been pre-treated, has been used in breathing gas filters to enhance removal of various toxic perfluorocarbons as is disclosed in U.S. Pat. No. 5,462,908. There is no disclosure as to the composition of the treated material or the nature of the nucleophile used to form a stable immobilized adduct with fluoroalkenes.
U.S. Pat. No. 6,521,461 (Mueller et al.) describes a method for removing one or more fluorinated alkenes from a fluid by contacting the fluid with an N-, S-, or P-containing nucleophile for a time sufficient to form an N-, S-, or P-containing nucleophile-fluoroalkene adduct. The nucleophile, and therefore the adduct, can be covalently bonded, coated or adsorbed to a particulate support which can be enmeshed in a porous, fibrous web.
Hexafluoropropylene (HFP) has been used in the preparation of a wide variety of useful fluorinated compounds and polymers, which are prepared by the fluoride-catalyzed addition of hexafluoropropylene to an electrophile. However, these methods invariably lead to the HFP dimeric and other oligomeric byproducts which can contaminate the desired product, may cause health or exposure risks, and are difficult to remove. Known means to remove the oligomeric byproducts are often expensive, time-consuming or involved the use of hazardous reagents. Simple distillation is often difficult due to the similar boiling points between the desired product and the undesired dimers. Distillation may also concentrate the toxic dimers and dimer hydrides. More particularly, many of the current methods are reactive with the desired reaction product as well as the undesired dimer byproduct and can reduce the yield of the desired, valuable products.
Briefly, the present invention provides a method of removing hexafluoropropylene dimers (xe2x80x9cHFP dimersxe2x80x9d), dimer hydrides and other oligomers from a fluid by heating the fluid to isomerize the HFP dimers to the thermodynamic isomer, and contacting the fluid with a tertiary amine (or salts thereof, preferably HF salts thereof) to form a hexafluoropropylene dimerxe2x80x94tertiary amine adduct. The method may further comprise the step of separating the dimer adducts from the reaction mixture.
The present invention provides a simple method of removing the undesired oligomeric byproducts using readily available materials and equipment. Advantageously, the method does not involve potentially hazardous reagents or reaction conditions, and the reagents do not substantially reduce the yields of the desired HFP-derived products.
The method is particularly useful in removing HFP dimers from fluorinated compounds derived from hexafluoropropylene, such as fluorinated ketones, HFP trimers and fluorinated aromatic compounds. Thus, the invention further provides a method of producing a fluorinated compound derived from hexafluoropropylene comprising the steps of contacting HFP with an electrophile in the presence of fluoride ion, optionally separating the crude reaction product, heating the crude reaction product to isomerize the HFP dimers, and treating the isomerized dimers with a tertiary amine (or salts thereof).
If desired, the tertiary amine may be adsorbed on, or coated on, or bonded to a support such as a porous on nonporous support. Useful supports may include a particulate support, a porous on nonporous film or web, or a foam.
As used in this application:
xe2x80x9cAdductxe2x80x9d or xe2x80x9cdimer-amine adduct,xe2x80x9d means the addition product of a tertiary amine and an HFP dimer with or without the elimination of a byproduct;
xe2x80x9cfluidxe2x80x9d refers to a material that is a liquid at 25xc2x0 C. and 760 mm Hg pressure, i.e., standard conditions;
xe2x80x9cHFP dimerxe2x80x9d refers to unsaturated compounds of the formula C6F12, formed by the dimerization of hexafluoropropylene.
xe2x80x9cHFP dimer hydridexe2x80x9d or xe2x80x9chydrides of HFP dimerxe2x80x9d refers to compounds of the formula C6F13H, formed by the addition of HF to an HFP dimer.
xe2x80x9cHFP oligomerxe2x80x9d refers to dimers, trimers and tetramers of hexafluoropropylene;